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Archive for July 2nd, 2014

Kenmore 158: Stepper Motor Max Speeds

Having a NEMA 23 stepper fit almost exactly into the spot vacated by the sewing machine’s AC motor was too good to pass up:

Kenmore 158 - NEMA 23 stepper - on adapter

Kenmore 158 – NEMA 23 stepper – on adapter

So I wired a power supply to an M542 stepper driver brick, connected the pulse output of a function generator to the brick’s STEP inputs, swapped motor leads until it turned the proper direction (CCW as seen from the shaft end), and turned the function generator knob:

Kenmore 158 - NEMA 23 stepper test

Kenmore 158 – NEMA 23 stepper test

The object was to find the step frequency where the motor stalls, for various winding currents and supply voltages. The motor won’t have enough torque to actually stitch anything near the dropout speed, but this will give an indication of what’s possible.

With a 24 V DC supply and 1/8 microstepping (40 k step/s = 1470 RPM):

  • 1.00 A = 11 k step/s
  • 1.91 A = 44 k/s
  • 2.37 A = 66 k/s
  • 3.31 A = 15 k/s

With a 36 V DC supply and 1/8 microstepping:

  • 1.91 A = 70 k/s
  • 3.31 A = 90 k/s

With a 36 V DC supply and 1/4 microstepping (40 k step/s = 2900 RPM):

  • 1.91 A = 34 k/s
  • 2.37 A = 47 k/s
  • 2.84 A = 47 k/s
  • 3.31 A = 48 k/s

The motor runs faster with a higher voltage supply, which is no surprise: V = L di/dt. A higher voltage across the winding drives a faster current change, so each step can be faster.

The top speed is about 3500 RPM; just under that speed, the motor stalls at the slightest touch. That’s less than half the AC motor’s top speed under a similarly light load and the AC motor still has plenty of torque to spare.

90 k step/s at 1/8 microstepping = 11 k full step/s = crazy fast. Crosscheck: 48 k step/s at 1/4 microstepping = 12 k full step/s. The usual dropout speed for NEMA 23 steppers seems to be well under 10 k full step/s, but I don’t have a datasheet for these motors and, in any event, the sewing machine shaft provides enough momentum to keep the motor cruising along.

One thing I didn’t expect: the stepper excites howling mechanical resonances throughout its entire speed range, because the adapter plate mounts firmly to the cast aluminum frame with absolutely no damping anywhere. Mary ventured into the Basement Laboratory to find out what I was doing, having heard the howls upstairs across the house.

She can also hear near-ultrasonic stepper current chopper subharmonics that lie far above my audible range, so even if the stepper could handle the speed and I could damp the mechanics, it’s a non-starter for this task.

Given that the AC motor runs on DC, perhaps a brute-force MOSFET “resistive” control would suffice as a replacement for the carbon disk rheostat in the foot pedal. It’d take some serious heatsinking, but 100 V (or less?) at something under 1 A and intermittent duty doesn’t pose much of a problem for even cheap surplus MOSFETs these days.

That would avoid all the electrical and acoustic noise associated with PWM speed control, which counts as a major win in this situation. Wrapping a speed control feedback loop around the motor should stiffen up its low end torque.

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